Optimization of statistical movement measurement for optical mouse, with particular application to laser-illuminated surfaces

ABSTRACT

A method for measuring relative motion between an illuminated portion of a surface and an optical sensing device comprising a coherent light source and a photodetector device comprising an array of pixels and comparators for extracting motion features, said method comprising the steps of:
         a) illuminating by means of said coherent light source said surface portion at a determined flash rate;   b) detecting by means of said array of pixels a speckled light intensity pattern of said illuminated portion of the surface for each flash;   c) extracting edge direction data of two different types from said detected speckled light intensity patterns by comparing light intensity between pixels;   d) determining a measurement of the relative motion between said optical sensing device and said illuminated portion of the surface based on extracted edge direction data;       

     wherein the extracting edge direction data step comprises a preliminary step consisting of introducing a selecting factor which promotes detection of one type of edge direction data rather than the other type.

FIELD OF THE INVENTION

The present invention generally relates to pointing devices, in particular for controlling the position of a cursor on a screen, such as the display of a personal computer, workstation or other computing devices having a graphic user interface. Such pointing devices may for instance include mice, trackballs and other computer peripherals for controlling the position of a cursor on a display screen.

The present invention more particularly relates to the field of optical pointing devices which comprise an optical motion sensing device including a photodetector array for measuring the varying intensity pattern of a portion of a surface which is illuminated with radiation of a laser illuminated source and for extracting information about the relative motion between the photodetector array and the illuminated portion of the surface.

BACKGROUND OF THE INVENTION

Optical pointing devices incorporating a laser illuminated source are already known in the art. Such laser illumination allows optical pointing devices such as mice to work on a much wider variety of surfaces. However the coherent nature of the illumination results in a received image that contains generally high spatial frequencies especially compared to viewing the same surface with a non coherent LED illumination. This high frequency content leads to spatial aliasing due to beyond under-sampled (in the spatial domain). This aliasing leads to several bad effects, such as loss of resolution or apparent “reverse” motion (the “wagon-wheel in motion pictures” effect). One alternative to deal with the higher spatial frequency content is to create smaller pixels (higher spatial capability in the imager). But, two major problems arise with smaller pixels that are a lower mouse-speed/acceleration capability and a lower sensitivity to light (less collection area in the pixel).

U.S. patent application Ser. No. 11/165,537, filed in the name of the same Assignee and which is incorporated in its entirety herein by reference, for instance discloses a method for measuring relative motion between an illuminated portion of a surface and an optical sensing device comprising a coherent light source and a photodetector array, the method comprising the steps of:

-   -   illuminating under a determined gradient by means of the         coherent light source the surface portion at a determined flash         rate;     -   detecting by means of the photodetector array speckled light         intensity pattern of the illuminated portion of the surface for         a first flash;     -   detecting a second speckled light intensity pattern of the         illuminated portion of the surface for a second flash;     -   extracting motion features of two different types from the         detected first and second speckled light intensity patterns;     -   keeping only pairs of neighbouring motion features including one         motion feature of both different types;     -   determining a measurement of the relative motion between the         optical sensing device and the illuminated surface portion based         on a comparison of kept motion features.

Although such a solution as disclosed in U.S. patent application Ser. No. 11/165,537 presents several advantages in dealing with a coherent light source in an optical pointing device, since laser illumination has a very large spectral content, there is still a need to further and better control the quantity of data needed to determine a measurement of relative between the optical pointing device and the illuminated surface portion.

SUMMARY OF THE INVENTION

One goal of the present invention is thus to implement a simpler and more reliable method for measuring relative motion between an illuminated portion of a surface and an optical motion sensing device comprising a coherent light source and a photodetector device comprising an array of pixels and comparators for extracting motion features.

For that purpose, the method comprises the steps of:

a) illuminating by means of said coherent light source said surface portion at a determined flash rate;

b) detecting by means of said array of pixels a speckled light intensity pattern of said illuminated portion of the surface for each flash;

c) extracting edge direction data of two different types from said detected speckled light intensity patterns by comparing light intensity between pixels;

d) determining a measurement of the relative motion between said optical sensing device and said illuminated portion of the surface based on extracted edge direction data;

wherein the extracting edge direction data step comprises a preliminary step consisting of introducing a selecting factor which promotes detection of one type of edge direction data rather than the other type.

Since an inflection is detected as one type of edge followed by the opposite type, increasing the amount of edges of one type with respect to the other will lead to a decrease in the overall inflection count, and thus the density of inflections. It is preferably proposed to introduce an adjustable offset into the comparators in order to decrease the inflection count in the image if it exceeds a given value. There is a need to keep the total inflection density low enough to avoid “confusing” the inflection tracking logic that determines movement. The added offset to these edge detection comparators is adjusted to cause then to prefer “one” type of edge (positive/negative) over the other type.

Furthermore, other advantageous embodiments form the subject of the dependent claims.

A refinement can be made to mitigate one issue with the simplest implementation of this offset. That issue is due to the non-uniform illumination of the optical source. Generally the laser will illuminate with a “Gaussian” shaped beam, and so the image will naturally have a Gaussian roll-off convolved with the actual image from the surface. The edges of the roll-off confuse the offset algorithm, and so the action in those regions is not what is desired. On one edge, the number of inflections is decreased, and on the other edge the number is increased. To get around this, the offset is advantageously dependent on which image quadrant the edge-comparators are in.

For that purpose, said photodetector array of pixels is aligned along first and second axes and divided into four quadrants upper left, upper right, lower left and lower right and the selecting factor is introduced by adding to comparators along the first axis a positive offset for both left quadrants and a negative offset for both right quadrants, and by adding to comparators along the second axis a positive offset for both lower quadrants and a negative offset for both upper quadrants.

A further refinement can be made to deal with a further problem. The optical alignment of the laser illumination might not be perfect, and so the central point of the Gaussian illumination might not correspond to the central point of the pixel array. To handle this issue, the algorithm measures the central point of illumination (with respect to X and Y), and change the sign of the offset polarity of each edge comparator to effectively move the matrix central point to correspond to the illumination central point.

For that purpose, the method further comprises a step consisting of determining the central point of a Gaussian illumination of said coherent light source and wherein the four quadrants are determined in relation with the central point of the Gaussian illumination.

Alternatively, since the coherent light source illuminates with a Gaussian shaped beam, the selecting factor is introduced by adding to said comparators an offset being adjusted in function of the Gaussian roll-off band. It may be further provided that the offset is inverted according to the Gaussian roll-off sign.

Since the algorithm reads the inflection count (along both axes X and Y) during every “flash” of the laser. A number of flashes (N) is averaged to produce an average inflection count (for X and Y), and the larger of the X or Y count is checked against a defined window. Then, the algorithm adjusts the offset voltage so that the count will stay within the defined window.

Additional enhancements are included to better handle various “end” conditions. For example, while the highest count of X or Y controls the algorithm, the other lower count of X or Y is also checked to make sure that it does not drop below a certain threshold. If it does, the offset may not be adjusted further.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features and advantages of the present invention will be apparent upon reading the following detailed description of non-limiting examples and embodiments made with reference to the accompanying drawings.

FIG. 1 represents a generalized schematic bloc diagram of an optical pointing device;

FIG. 2 shows a three-dimensional image of the laser illumination which has a fairly steep Gaussian gradient;

FIG. 2 a represents a cut section of the plan A-A in FIG. 2;

FIG. 2 b represents a photodetector array cut into four quadrants;

FIG. 3 is a schematic illustration of edge inflection conditions, or peaks and nulls, derived from a sequence of edge direction conditions along a determined axis of the photodetector array.

DETAILED DESCRIPTION OF THE INVENTION

The following description, which concerns a method for measuring relative motion between an illuminated portion of a surface and an optical sensing device comprising a coherent light source and a photodetector comprising an array of pixels and comparators for extracting motion features, is given by way of a non limiting example in relation with FIGS. 1 to 3.

Algorithms that may be used for determining the measurement of the relative motion between the optical sensing device and the illuminated portion of the surface based on extracted edge direction data are given by way of example in US Patent Application Publication No 2005/0062720 filed in the name of the same Assignee and enclosed herewith in its entirety by way of reference. It is understood that various adaptations may be done on these algorithms.

According to these algorithms, extracted motion features are defined as edge direction data given by comparing light intensity between pixels, namely a first edge condition, or positive edge, defined as a condition wherein the light intensity of a first pixel is less than the light intensity of a second pixel, and a second edge condition, or negative edge, defined as a condition wherein the light intensity of the first pixel is greater than the light intensity of the second pixel.

Further, referring to the “Peak/Null Motion Detection” algorithm each row and column of the photodetector array are further analysed to find specific inflection conditions (hereinafter defined as a first inflection condition, or “peak”, and a second inflection condition, or “null”) in the direction of successive edges along a selected axis (in practice along both the X and Y axes). As illustrated in FIG. 3, the first inflection condition, or peak, is defined as the succession, along a determined axis (X or Y), of a positive edge (arrow pointing upwards in FIG. 3) followed by a negative edge (arrow pointing downwards in FIG. 3). Similarly, the second inflection condition, or null, is defined as the succession, along the determined axis, of a negative edge followed by a positive edge.

Considering now FIG. 1, it represents a generalized schematic bloc diagram of an optical pointing device. It comprises a photodetector array 100 including a plurality of pixels, this photodetector array 100 being coupled to processing means 110 (or motion detection processing circuit) for processing the signals outputted by the photodetector array 100.

A comparator array 120 may be interposed between processing means 110 and array 100, this comparator array 120 including a plurality of comparator circuits each for comparing the light intensity of a first pixel of array 100 with the light intensity of a second pixel of array 100 and for outputting resulting motion feature conditions.

The optical pointing device further comprises at least one coherent light source 130 such as a laser illumination source, which produces radiation at a determined flash rate, that impinges with a determined gradient on a portion of a surface S. Surface S may be a planar or non-planar surface, such as a surface over which the pointing device is moved (as in the case of an optical mouse), the surface of a ball (as in the case of an optical trackball) or any other suitable surface that may provide an appropriate speckled intensity pattern for detection by photodetector array 100.

Processing means 110 is further adapted to communicate in a bidirectional manner with an interface 140 that communicates in turn with a host system (not illustrated) over a bus 150. Cursor control signals (and eventually other signals related to the optical pointing device) are supplied to the host system over bus 150. Processing means 110 may also receive information, such as configuration signals, over bus 150 from the host system.

Processing means 110 is essentially designed to intermittently sample the pixel outputs of photodetector array 100 in accordance with a defined sequence. The information of two successive samples or speckled images is compared and a relative motion measurement is extracted by processing means 110. The adequate cursor control signals are then derived from the relative motion measurement and transmitted to the host system via line interface 140.

However, as it has been already mentioned in the introduction of the specification, the light intensity pattern detected by photodetector device 100 forms a speckled image which presents to many motion features which render motion detection less reliable. For that purpose according to the present invention, processing means 110 are provided with a very simple selecting factor 160 which promotes detection of one type of motion feature rather than the other type. Such simple selecting factor will be explained and better understood below in relation with FIGS. 2, 2 a and 2 b.

FIG. 2 shows a three-dimensional image of the laser illumination which has a fairly steep Gaussian gradient. The plan defined by axes X and Y represents the photodetector array and axis Z represents the detected light intensity. As can be easily seen on this image, the light intensity increases or decreases according to the quadrant and axis considered. Thus if one cuts the array of pixels in four quadrants (as shown on FIG. 2 b), namely, the upper left, upper right, lower left and lower right quadrants, the light intensity (LI) departing from edges to the centre of the array of pixels will change in the following manner:

in the upper left quadrant, the light intensity increases along both X and Y axes;

in the upper right quadrant, the light intensity increases along X axis and decreases along axis Y;

in the lower left quadrant, the light intensity decreases along the X axis and increases along the Y axis; and

in the lower right quadrant, the light intensity decreases along both X and Y axes.

As mentioned previously, the purpose of the algorithm is to control the average inflection count that the sensor sees. Since laser illumination has a very large spectral content, it leads to a dense inflections image, which causes the motion detection algorithms of the sensor to alias. The aliasing is usually seen as degradation of resolution with speed and/or acceleration detection (loss-of-tracking event) failure. By controlling the average inflection count, one also controls indirectly the density of the inflections image and aliasing may be prevented.

By adding an adjustable offset to the edge detection comparators in function of the quadrant, the inflection count is decreased. Actually, adding an offset to the edge detection comparators causes them to “prefer” one type of edges (positive/negative) over the other. Since an inflection is detected as one type of edge followed by the opposite type, increasing the amount of edges of one type on the account of the other will necessarily lead to a decrease in the inflection count.

For that purpose, according to an embodiment of the present invention and in accordance with the quadrant considered, the offset is adjusted in the following manner:

a negative offset for both lower quadrants along the X axis;

a positive offset for both upper quadrants along the X axis;

a positive offset for both left quadrants along the Y axis; and

a negative offset for both right quadrants along the Y axis.

Since the central point of the Gaussian illumination of the coherent light source is not always centered with respect to the array of pixels, the four quadrants are preferably determined in relation with the actual central point of the Gaussian illumination.

According to another embodiment of the present invention, since the coherent light source illuminates with a Gaussian shaped beam, the selecting factor is introduced by adding to the edge comparators an offset being adjusted in function of the Gaussian roll-off band. Preferably the offset is inverted according to the Gaussian roll-off sign.

Advantageously, the algorithm reads the X and Y inflection count for every flash. N flashes are averaged to produce X and Y average inflection count. For instance, the number of consecutive flashes used to calculate the average inflection count; may by default be fixed to 4. The algorithm adjust the offset voltage, so the larger between X and Y average inflection count falls within a pre-defined window of inflection count. The window is defined for both axis X and Y, with targeted maximum and minimum average inflection counts, respectively NinfAveHi and NinfAveLo.

The offset control algorithm will adjust the comparators offset so to the higher average inflection count (the higher between X average inflection count and Y average inflection count) will fall within the window that is defined by minimum and maximum values NinfAveLo and NinfAveHi. Thus if the higher inflection count is larger than the maximum average inflection count NinfAveHi, the offset voltage will be increased. If the higher average inflection count is smaller than the minimum average inflection count NinfAveLo, the offset voltage will be decreased.

Since the algorithm is looking at the higher average inflection count, a precaution should be taken regarding the lower inflection count. One would not want this inflection count to go below some level (an extremely low inflection count also leads to performance degradation). Thus, the algorithm allows an increase of the offset voltage if the higher average inflection count is larger than the maximum average inflection count NinfAveHi and if the lower average inflection count is higher than a targeted absolute minimum inflection count NinfAveMin.

FIG. 2 a represents a cut section of the plan A-A in FIG. 2. Adding a positive offset when the light intensity increases has the effect to accentuate the light intensity increase due to the Gaussian shaped beam and eliminate succession of positive and negative edges, i.e. inflection, due to small intensity differences and therefore reduce the total number of the inflection count. 

1. A method for measuring relative motion between an illuminated portion of a surface and an optical sensing device comprising a coherent light source and a photodetector device comprising an array of pixels and comparators for extracting motion features, said method comprising the steps of: a) illuminating by means of said coherent light source said surface portion at a determined flash rate; b) detecting by means of said array of pixels a speckled light intensity pattern of said illuminated portion of the surface for each flash; c) extracting edge direction data of two different types from said detected speckled light intensity patterns by comparing light intensity between pixels; d) determining a measurement of the relative motion between said optical sensing device and said illuminated portion of the surface based on extracted edge direction data; wherein the extracting edge direction data step comprises a preliminary step consisting of introducing a selecting factor which promotes detection of one type of edge direction data rather than the other type.
 2. The method according to claim 1, wherein said array of pixels is aligned along first and second axes and divided into four quadrants upper left, upper right, lower left and lower right and wherein said selecting factor is introduced by adding to comparators along the first axis: a positive offset for both left quadrants; a negative offset for both right quadrants; and by adding to comparators along the second axis: a positive offset for both lower quadrants; a negative offset for both upper quadrants.
 3. The method according to claim 2, wherein it further comprises a step consisting of determining the central point of a Gaussian illumination of said coherent light source and wherein said four quadrants are determined in relation with said central point of the Gaussian illumination.
 4. The method according to claim 1, wherein said coherent light source illuminates with a Gaussian shaped beam and wherein said selecting factor is introduced by adding to said comparators an offset being adjusted in function of the Gaussian roll-off band.
 5. The method according to claim 4, wherein said offset is inverted according to the Gaussian roll-off sign.
 6. The method according to claim 2, wherein an inflection is detected as one type of edge followed by the opposite type, wherein the method further comprises the steps of: counting along both axes the number of inflections for each flash; averaging the inflection count of a determined number of flashes for each axes; comparing the higher inflection count of either the first or the second axis with a defined average inflection count window; adjusting the offset voltage so that the count will stay within the defined window.
 7. The method according to claim 6, wherein the window is defined by minimum and maximum average inflection count values and wherein if the higher inflection count is greater than the maximum average inflection count value, the offset voltage is increased, and if the higher average inflection count is smaller than the minimum average inflection count, the offset voltage is decreased.
 8. The method according to claim 7, wherein a targeted absolute minimum inflection count is defined and wherein the offset is increased if the higher average inflection count is greater than the maximum average inflection count and if the lower average inflection count is greater than said absolute minimum inflection count. 